A majority of new cases of HIV are a result of sexual transmission between an HIV negative and an HIV positive individual. To fully understand the context of HIV transmission, we need to understand the behavior of both parties. Yet most of is known about the sexual behavior of HIV positive people is based on small or unrepresentative samples recruited as part of larger samples of persons at risk. The HIV Cost and Services Utilization Study (HCSUS), now being conducted under a cooperative agreement between the Agency for Health Care Policy and Research and RAND, is currently studying a national probability sample of HIV positive persons who are receiving health care. The sample includes men and women from diverse backgrounds and multiple risk groups. Although designed for the purpose of obtaining nationally representative data on HIV positive persons in care and their providers, HCSUS provides an unprecedented, one-time opportunity to examine sexual behavior in a sample that has been carefully designed to provide unbiased estimates for the entire population of HIV positive people in care. This two-year study will examine the sexual and risk reduction behaviors reported by HCSUS participants to gain a better understanding of the prevalence of these behaviors and the context in which they occur. The study will examine sexual behaviors that can transmit HIV and safer sex practices that reduce risk; barriers and incentives to practicing safe sex; psychosocial dynamics; attitudes and perceptions relevant to risk reduction; fertility plans and attitudes. Data will be obtained from personal interviews with about 1,838 adults being followed as part of HCSUS. In addition, linked data from the core HCSUS study will be available for supplementary analysis.